Investigation of Microstructure and Stress Corrosion Cracking in Al-6061-T6 Alloy at Different Loads

Abstract

Stress corrosion cracking (SCC) refers to the damage of mechanical components which are under the combined action of static load and corrosive environment. This phenomenon occurs in various applications including naval and aerospace industry where aluminum and steel alloys experience mechanical loadings in the presence of corrosive environments. In this research work, microstructural and environmental influence on corrosion behavior of Al-6061-T6 at different static loads was investigated. A new test fixture was developed for stress corrosion cracking. Dog-bone shaped tensile specimens of Al-6061-T6 were manufactured using CNC milling machine. Tests were conducted at constant loads of 200 N, 500 N and 800 N, in three different environments: dry ambient conditions, distilled water and 3.5% NaCl solution. Testing continued for different intervals of time i.e. 96 hours, 68 hours and 4.5 hours respectively. After each set of experiments, specimens were observed for cracks using metallurgical microscope. Detailed fractographic investigation of all the tested specimens was carried out using Scanning Electron Microscope (SEM). Excessive corrosion and material degradation was observed in specimens tested in distilled water and 3.5% NaCl environments. Microstructural analysis depicted pitting corrosion and crack deformation. Some regions clearly showed that grain boundaries were attacked due to oxidation and chemical attack causing weakening of grain boundaries and resulted into intergranular corrosion. Precipitates and grain boundaries in Al-6061-T6 served as a reason of crack initiation due to hydrogen diffusion. Fractographic investigation provided the evidence of trans granular fracture as well as intergranular fracture which was observed as dimples and extensive ductile tearing.